1. Field of the Invention
The present invention is concerned with aqueous gluten-containing compositions useful for coating of paper stock, allowing the coated stock to be printed to obtain high quality, high gloss end products. More particularly, the invention is concerned with such compositions as well as methods of preparation and use thereof, where preferred compositions include reduced wheat gluten and wheat starch dispersions having relatively low viscosities up to about 2000 cP; these compositions are water soluble, so that the compositions and printing thereon may be easily removed from the paper stock by soaking in water, thereby facilitating repulping of the stock.
2. Description of the Prior Art
The paper and board industries are constantly searching for better ways to make coated paper and board with improved quality at reduced cost. Thus, coatings are commonly applied to paper stock in an attempt to make the surfaces of the paper stock conducive to printing using high speed web fed printing equipment by improving the smoothness, gloss, ink printing sharpness, drum adhesion and pick resistance of the stock. Known paper coating formulations include latex or other synthetic resin materials. The generally lower viscosity and water binding capabilities of these coatings allows applications of high solids coating layers onto paper stock. While these formulations do improve the surface properties of the paper stock, but they are very expensive and difficult to use. Moreover, latex/synthetic resin coatings are hard to remove, making de-inking and de-waxing of the stock very difficult; this in turn prevents effective repulping of the stock at a reasonable cost.
Gelatinized, hydrolyzed, and other modified starches have also been used in prior art paper coating compositions. However, the viscosities of these starches are so high that the solids content of the formulations must be limited. This results in compositions which do not adequately coat the paper. Furthermore, the unpredictable behavior of starches such as corn starch, wheat starch, and potato starch typically leads to inconsistent coating properties, particularly at high coating speeds.
Producers of consumer packaging products or other paper stock items requiring high quality, high gloss printing have almost without exception been forced to use relatively expensive grades of paper stock. Kraft stock is readily available and is much less expensive than other types of paper. However, it has heretofore been virtually impossible to use Kraft where high quality printing is needed, owing to the tendency of printing inks to absorb into and smear on Kraft.
There is accordingly a real and heretofore unresolved need for improved compositions which can be used to form smooth, high quality coatings on paper products On the one hand a successful coating composition must permit sharp printing of ink images at high printing speeds using conventional printing equipment. On the other hand, such a coating must be relatively low cost and not present undue application problems. Finally, given the increasing concern about recycling of paper products, an improved coating must not interfere with, and preferably should enhance, the ability to repulp the coated paper products after use thereof.
The present invention overcomes the problems outlined above, and provides coating compositions which can be used to coat paper stock of virtually any kind (and particularly relatively inexpensive Kraft stock), so that the coated stock can be printed using conventional equipment to obtain a final printed product of high quality with heretofore unobtainable gloss values. Moreover, the preferred compositions are water soluble to facilitate repulping of the coated and printed stock.
In more detail, the coating compositions of the invention are in the form of aqueous dispersions including therein respective quantities of filler and wheat gluten. The filler is normally present in the dispersion at a level of from about 20-45% by weight, and more preferably from about 25-35% by weight, based upon the total weight of the dispersion taken as 100% by weight. The filler can comprise a mineral filler, a starch, or mixtures thereof. Preferred mineral fillers include clay (#1 and #2), calcium carbonate (ground or precipitated), talc, and mixtures thereof, while preferred starch fillers include wheat starch, corn starch, potato starch, rice starch, tapioca starch, modified versions of these starches (e.g., hydroxypropylated, acetylated, crosslinked, oxidized, cationized, acid-thinned starches), and mixtures thereof.
The wheat gluten should be present in the dispersion at a level of from about 6-18% by weight, and more preferably from about 6-12% by weight, based upon the total weight of the dispersion taken as 100% by weight. The gluten may be derived from commercially available wheat glutens of varying grades.
As used herein, xe2x80x9cglutenxe2x80x9d or xe2x80x9cwheat glutenxe2x80x9d refers to native and/or modified wheat glutens of various types. For example, wheat gluten may be modified by reducing agent(s) as hereafter described. However, other wheat gluten modifications, either in addition to or in lieu of reducing agent treatment can be used. Thus, wheat gluten may be oxidized, acylated, alkylated, deaminated or hydrolyzed (with a degree of protein hydrolysis usually less than 1%) or subjected to combined treatments.
As indicated, the preferred wheat gluten is initially modified with a reducing agent so as to cleave at least some of the disulfide bonds therein (preferably at least about 5%, and more preferably from about 10-100% of the disulfide bonds) and reduce the average molecular weight of the gluten. Thus, the gluten utilized preferably has a weight average molecular weight of less than about 1,000 kDa, more preferably less than about 500 kDa, and most preferably from about 20-60 kDa.
The gluten reducing agent is preferably added to the dispersion at a level of from about 0.05-2.0% by weight, and more preferably from about 0.1-1.0% by weight, based upon the total weight of the gluten taken as 100% by weight. Preferred reducing agents include alkali metal sulfites, alkali metal bisulfites, alkali metal metabisulfites, sulfur dioxide, mercaptan, and cysteine, with sodium metabisulfite being the most preferred reducing agent.
The compositions should have a Brookfield viscosity (determined on an RVT model equipped with a #2 spindle; 100 rpm; 73-74xc2x0 F.) of less than about 2000 cP, preferably less than about 500 cP, and more preferably from about 60-150 cP. Furthermore, the solids content of the dispersion is preferably from about 25-57% by weight, and more preferably from about 30-50% by weight, based upon the total weight of the dispersion taken as 100% by weight. Finally, the finished compositions should have a pH of from about 9-12, and more preferably from about 9.5-11.
Preferably, the compositions have a weight ratio of filler:wheat gluten of from about 3:1 to about 5:1, and more preferably from about 3:1 to about 4:1. Normally, the preferred dispersions made up of wheat gluten and wheat starch are formulated using initially separate starch and gluten, i.e., they are not both derived from a single wheat flour or the like. In another embodiment, the preferred compositions consist essentially of aqueous dispersions including therein starch (and especially wheat starch), wheat gluten, a reducing agent, and a base (e.g., NaOH).
The compositions may be formed by preparing an aqueous dispersion of water and gluten, and may also include from about 0.1-0.5% by weight of a defoamer (such as a silicone defoamer), based upon the total weight of all ingredients utilized taken as 100% by weight.
Thereupon, a base such as NaOH is mixed with the dispersion in sufficient amounts to yield a dispersion pH of from about 10-12, and more preferably from about 11.5-11.7. The base is typically mixed with the dispersion at a level of from about 1-3% by weight, and preferably from about 1.5-2.5% by weight, based upon the total weight of the gluten utilized taken as 100% by weight. After base addition, the reducing agent is mixed with the dispersion so as to cleave disulfide bonds in the gluten.
Next, a starch such as Midsol 50 wheat starch (available from Midwest Grain Products, Atchison, Kans.) is added to the gluten dispersion in the ratio of 3:1 to about 5:1 as a filler. Final pH of dispersion at this stage is in a range of 9.0-12.0, but preferable from 10.0 to 10.8.
In alternate forms, a dry mixture of wheat gluten, starch and a reducing agent (e.g., sodium metabisulfite) can be provided. As needed, this dry mixture may be processed by the addition of water in a vacuum dissolver so as to simultaneously reduce the gluten and provide the necessary mixing to create the use dispersion. The latter may be used directly by application to paper stock, or can be stored for future use. In such an embodiment, the starch:gluten ratio should be from about 3:1 to 5:1. The gluten is normally present at a level of from about 16-25% by weight of the dry composition, the starch is present at a level of from about 75-85% by weight of the dry composition, and the reducing agent is present at a level of from about 0.05-1% by weight of the wheat gluten. In the mixing procedure, an appropriate amount of the dry composition is added to water to achieve the above-described amounts of ingredients in a flowable coating composition.
The compositions hereof can be applied to paper stock (as used herein, paper stock is intended to include all forms, types and weights of paper such as Kraft stock). Conventional coating equipment can be employed for this purpose. Normally, in order to achieve the best quality coated stock, the compositions are applied as separate coatings or xe2x80x9cbumpsxe2x80x9d, with intermediate partial or complete drying and curing of the compositions. The final coated stock can be printed using normal web-fed printing equipment and conventional inks. A particular feature of the coated stock of the invention is that the final printing can achieve very high gloss values on the order of from about 50-65 units. Furthermore, the coated stock will give a passing result when subjected to Testing Association for the Pulp and Paper Industry (TAPPI) Useful Method 557 as described herein (also known as the xe2x80x9c3Mxe2x80x9d test).
Another feature of the invention is that the coated stock may be readily repulped. That is, the compositions of the invention are preferably water soluble, so that when the coated/printed stock is placed in water the coating and ink layers come off as a skimmable layer, leaving the stock ready for conventional repulping. Thus, previously hard to repulp products such as polyether wax treated poultry boxes can be easily repulped if the paper stock is first coated with compositions in accordance with the invention prior to application of wax and printing.